Numerous "radiological examination procedures" already directly provide "radiological images", suitable for diagnostic evaluation, in digital form. Hereinafter the term "radiological examination procedures" has to be understood as those examination procedures that give an image of the interior of a body irrespective of the ways in which said image is created. E.g. ultrasonography, medical thermography, magnetic resonance imaging, positron emission tomography (PET), etc are, for the understanding of the present invention, included, together with procedure using X-rays, in the term radiological examination procedures. The term "radiological image" has to be understood as the image generated by said "radiological examination procedures" and the term "radiological department" has to be understood as this department of a hospital or as a private practice where "radiological examination procedures" are performed.
Examples of radiological examination procedures directly providing images, suitable for diagnostic evaluation, in digital form include digital subtraction angiography, magnetic resonance imaging, computer aided tomography, computed radiography etc. Computed radiography is described in, e.g., U.S. Pat. No. 3,859,527, where an X-ray recording system is disclosed wherein photostimulable storage phosphors are used having, in addition to their immediate light emission (prompt emission) upon X-ray irradiation, the property to store temporarily a large part of the absorbed X-ray energy. Said energy is set free by photostimulation in the form of fluorescent light different in wavelength from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals. This recording method gives an X-ray image in digital form.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed in an automatic developing machine to form therein a silver image in conformity with the X-ray image. The analog image which is recorded in said photographic silver halide emulsion layer can be converted into a digital form either by digitizing said analog image after diagnosis or by digitizing said analog image directly when it sorts out of said developing machine. Means for directly digitizing analog X-ray images recorded on silver halide emulsion layers are described in e.g. EP-A 452571.
In many "radiological examination procedures" which deliver a "radiological image" in digital form, sensors having a good signal to noise ratio over a large dynamic range. This is especially the case with computed radiography and computed tomography.
While the diagnosis is preformed by a human observer, the digital image as obtained, containing diagnostically important information within a wide amplitude range, has to be represented in a human readable (analog) form. This is done by representing the image on a film hardcopy (to be viewed on a lightbox) or on a display screen. In both case the contrast of anatomic detail, as present in the digital image, must always be traded of against dynamic range of the medium on which said digital image is represented. Given the limited dynamic range of the image output medium (smaller than 500:1 in case of a transparent film, and smaller than 100:1 in case of a CRT screen under normal viewing conditions) then the tradeoff can be stated extremely as follows:
i) if the entire dynamic range of the diagnostically meaningful signal levels is mapped onto the available output medium dynamic range, then overall contrast will be very low, and for many subtle, diagnostically important details, the image contrast will be below the perceptual threshold level, hence these will be missed by the observer. PA1 ii) if at the other hand only a part of the original dynamic range is mapped onto the output medium dynamic range then all signal levels below this range will all be mapped onto the same (low) output level, and all original levels exceeding this range will be mapped onto the same (high) output level. PA1 (i) recording said image directly in an digital form or recording said image as an analog image and transforming said analog image into a digital image, PA1 (ii) determining a raw image histogram of said digital image PA1 (iii) determining from said histogram the width of a diagnostically useful window PA1 (iv) dividing said useful window into several smaller windows, the width of said smaller windows being adapted to the dynamic range of said recording medium PA1 (v) feeding digital image data of each of said smaller windows to an imager PA1 (vi) printing the information content of each of said smaller windows onto said image recording medium. PA1 (i) recording said image directly in an digital form or recording said image as an analog image and transforming said analog image into a digital image, PA1 (ii) determining a raw image histogram of said digital image PA1 (iii) determining from said histogram the width of a diagnostically useful window PA1 (iv) dividing said useful window into several smaller windows, the width of said smaller windows being adapted to the dynamic range of said recording medium PA1 (v) combining digital image data, describing said smaller windows, with digital text data of said protocol PA1 (vi) feeding said combined digital image data and digital text data to an imager PA1 (vii) printing said combined digital data onto a single sheet of said image recording medium.
In that case only those image pixels having a level within the selected dynamic range will be presented with acceptable contrast, while the other pixels will have uniform brightness, and will show up with no contrast at all.
In image workstations connected to a computed radiography or computed tomography system the desired compromise between both extreme mappings is interactively selectable, a feature which is commonly referred to as window/level setting. This problem is largely recognized in the field of digital radiology, see: Maack I., Neitzel U., "Optimized Image Processing for Routine Digital Radiography", Proceedings International Symposium CAR '91, p. 109, Springer Verlag. A possible solution to this problem is described in e.g. EP-A 527 525.
For making hardcopy images almost exclusively recording media comprising a transparent support are used, while the dynamic range of a recordium medium comprising an opaque, reflecting support is at best 80:1, and in some cases even only 50:1. When the entire dynamic range of the diagnostically meaningful signal levels is mapped onto the available dynamic range of a recording medium comprising an opaque, reflecting support, even more information will be lost.
Usually radiological examination procedures are performed in a radiological department of an hospital on demand of a doctor. This doctor can belong to an internal service of the hospital or can be a phisician working outside of the hospital and is called "the referring physician".
After diagnosis the diagnostician writes a protocol of his findings (i.e. text describing the diagnostillay relevant information that is contained in the radiological image) and sends copies of the radiological images together with said protocol to the referring physician.
Since the radiological image is printed on a recording medium with a transparent support, said physician needs a viewing box to view the radiological images. It would for the referring physician be more convinient if it would be possible to print radiological images onto a recording medium comprising an opaque reflecting support. Such a material would eliminating the need of a viewing box an make it more convenient for showing the radiological print to the patient. Moreover, on a recording material having an opaque reflecting support it is possible to have the radiological image and the protocol of the radiologist printed on the same sheet. Having both the radiological image and the protocol inseperably bounded together will avoid possible mix-ups between radiological images and protocols: the referring physician is always certain that protocol that he receives from the radiologist refers to the radiological image.
Using hard copies of radiological images on an opaque reflecting support has advantages both from the viewpoint of convenience and from the viewpoint of costs. Recording media on an opaque reflecting support are usually less expensive than recording materials on a transparent support and it is for the referring physician more convenient to show the radiological image to the patient when the referring physician does not need a viewing box to show said images.
Although it would be easier for the referring physician to have a radiological image printed on a recording medium comprising an opaque, reflecting support, it has up until now been necessary to print the radiological images onto a recording medium comprising a transparent support, since the printing on a recording medium comprising an opaque reflecting support to much of diagnostically relevant information may be lost.
There is thus still a need to provide the "referring physician" with "radiological images" printed together with the protocol describing the radiological image on a recording medium comprising an opaque reflecting support and there is still the need for a method that enables to print radiological images on a recording material, comprising an opaque reflecting support, without loss of diagnostically relevant information, especially if the original digital image is sensed by sensors having a good signal to noise ratio over a large dynamic range.